This invention relates to a highly doped zinc oxide coated glass article exhibiting high visible light transmittance with low total solar energy transmittance.
Coatings on architectural glass are commonly utilized to provide specific energy absorption and light transmittance properties. Additionally, coatings provide desired reflective or spectral properties that are aesthetically pleasing. The coated articles are often used singularly or in combination with other coated articles to form a glazing or window unit.
Coated glass articles may be produced “on-line” by continuously coating a glass substrate while it is being manufactured in a process known in the art as the “float glass process.” Additionally, coated glass articles are produced “off-line” through a sputtering process. The former process involves casting glass onto a molten tin bath which is suitably enclosed, thereafter transferring the glass, after it is sufficiently cooled, to lift out rolls which are aligned with the bath, and finally cooling the glass as it advances across the rolls, initially through a lehr, and thereafter, while exposed to the ambient atmosphere. A non-oxidizing atmosphere is maintained in the float portion of the process, while the glass is in contact with the molten tin bath, to prevent oxidation of tin. An oxidizing atmosphere is maintained in the lehr. In general, the coatings are applied onto the glass substrate in the float bath of the float bath process. However, coatings may also be applied onto the substrate in the lehr.
The attributes of the resulting coated glass substrate are dependent upon the specific coatings applied during the float glass process or an off-line sputtering process. The coating compositions and thicknesses impart energy absorption and light transmittance properties within the coated article while also affecting the spectral properties. Desired attributes may be obtainable by adjusting the compositions or thicknesses of the coating layer or layers. However, adjustments to enhance a specific property can adversely impact other transmittance or spectral properties of the coated glass article. Obtaining desired spectral properties is often difficult when trying to combine specific energy absorption and light transmittance properties in a coated glass article. It is also difficult to obtain useful film thicknesses as the available deposition time in on-line processes is mere seconds, as the continuous glass ribbon is moving at a speed of several hundred inches per minute.
Deposition of zinc oxide coatings is known from the patent literature.
U.S. Pat. No. 4,751,149 describes a method of applying a zinc oxide coating to substrate at a low temperature by using a mixture of an organozinc compound and water carried in an inert gas. The resulting zinc oxide film is said to have relatively low resistivity which can be varied by addition of a Group III element.
U.S. Pat. No. 4,990,286 describes zinc oxy-fluoride films produced by CVD from vapor mixtures of zinc, oxygen and fluorine-containing compounds. Electrical conductivity of the film is said to be increased by substituting fluorine from some of the oxygen in the zinc oxide. The resulting films are said to be transparent, electrically conductive and infrared reflecting.
U.S. Pat. No. 6,071,561 describes a method of depositing fluorine-doped zinc oxide films utilizing vaporized precursor compounds such as a chelate of dialkylzinc, more specifically utilizing an amine chelate, an oxygen source and a fluorine source. The coatings produced are said to be electrically conductive, IR reflective, UV absorbing and free of carbon.
U.S. Pat. No. 6,416,814 describes the utilization of ligated compounds of tin, titanium and zinc as CVD precursor compounds to form metal oxide coatings on heated substrates.
U.S. Pat. No. 6,858,306 describes a glass substrate having disposed thereon a multi-layer coating of an antimony doped tin oxide, and a coating of fluorine doped tin oxide. The glass substrate so coated exhibits low emittance and high solar selectivity, thus providing improved heat rejection in summer and heat retention in winter while still permitting the transmittance of a relatively high degree of visible light.
Deposition of highly doped zinc oxide has been reported in the scientific literature for use in, for example, solar cells. Some examples of such articles follow:
Park et al. report deposition of highly doped ZnO films via pulsed laser deposition (see Japanese Journal of Applied Physics, Vol. 44, No. 11, 2205, pp. 88027-31). Using aluminum as the dopant, samples with an electron concentration of 1.25×1021 cm−3 and an electron mobility of 37.6 cm2N-s were said to have been produced. The investigators state that doped zinc oxide films can be used as transparent contacts in solar cells, laser diodes, ultra-violet lasers, thin film resistors, flat panel displays, and organic light-emitting diodes.
Similarly, Singh et al. (Journal of Indian Institute of Science, Vol. 81, September-October 2001, pp. 527-533) describe deposition of highly doped ZnO by pulsed laser ablation. Using a zinc oxide target doped with 2% aluminum oxide, ZnO:Al samples with an electron concentration of 1.5×1021 cm−3 and an electron mobility of 29 cm−3/V-s were said to have been produced.
Das and Ray deposited aluminum doped zinc oxide films by rf-magnetron sputtering, the films obtained were said to exhibit an electron concentration of 2.3×1021 cm−3. (Journal of Physics D: Applied Physics, Vol. 36, 2003, pp. 152-5.)
Finally, Choi et al. deposited gallium doped zinc oxide by rf-magnetron sputtering and claims to have produced films exhibiting an electron concentration of 1.5×1021 cm−3. (Thin Solid Films, Vol. 192-4, 1990, pp. 712-720.)
It would be advantageous to provide a coated glass article having a neutral tint that rejects solar energy in the summer and provides a low U value for the winter. A solar reducing glazing with a low emittance, and a low total solar energy transmittance, would significantly improve energy costs in buildings and homes while providing a desirable neutral tint.
It would also be advantageous to provide a solar reducing glazing that has a neutral color in reflectance, a low emittance, a high visible light transmittance, and a low total solar energy transmittance. The use of such a neutral colored article in architectural glazings would permit the transmission of a high degree of visible light while rejecting a significant amount of near infrared energy. Furthermore, the low emittance characteristic of the glazing would minimize any indirect heat gain from absorption.